SKY WATCHERS are enjoying a bonanza. Having become accustomed to the nightly spectacle of Comet Hale Bopp, those with an interest in environmental matters will be out in force today to observe what is expected to be the "deepest ever" ozone hole to cross northern Europe. Given good weather, we in Ireland can expect a grandstand view.
For those interested in viewing this unusual spectacle, it will be visible from everywhere in Ireland high in the northern sky as a faint but clearly discernible "lightening" on the normal blue, or as an anomulous area of brightness where skies are cloudy. The affected zone will be elliptical and about three "moons" in diameter at its widest point.
Scientists advise would be observers to use good quality sunglasses, both to improve the optical contrast between the "hole" and the ozone rich sky surrounding it, and also to protect the eyes from the small but significant increase in ultra violet radiation present while the hole is overhead.
Special ozone and UV readings will he undertaken today at all Met Eireann's observing stations and it is expected the collected data will add considerably to our knowledge of the formation and decay of these phenomena.
Ozone is a chemical variant of oxygen which forms a protective layer some 15 miles above our heads to protect the planet from the worst effects of the sun's harmful ultra violet radiation. In 1985, over Antarctica, the first "ozone hole" was discovered a well defined region where ozone concentrations were reduced dramatically below their normal values. They are rare in the northern hemisphere, but form occasionally in spring when four conditions are simultaneously fulfilled.
First, there must be abundant man made chloroflourocarbons, or CFCs, in the high atmosphere. Secondly, the temperature in the stratosphere must be very low, usually approaching -80 Celsius, a condition met at winter's end in the northern latitudes of either hemisphere. The third requirement is the photochemical effect of sunlight which becomes available as the sun begins to peep over the horizon after the long polar night.
And the final requisite is the development of a vortex - a persistent quasi circular motion of the upper air. The vortex, resembling a whirlpool, has the effect of isolating the air inside it allowing the ozone depleting processes to take their course without dilution by ozone rich air from elsewhere.
The ozone hole currently being tracked across the North Atlantic formed about two weeks ago in a deep vortex in the circum polar airflow, and was first spotted during routine observations at Yahfulya Observatory in north west Canud.
It reached its largest dimensions on March 28th when over the southern tip of Greenland, forming an ellipse some loo km in length and 30 km wide near the centre of it ozone concentrations were reduced to 190 Dobson Units, or slightly less than 50 per cent of normal.
It has been following an undulating eastward track, travelling at an average speed of some 30 knots, and is predicted to pass just north of Ireland between about midday and 2 p.m. today. Because of its relatively rapid movement no harmful effects will result from the phenomenon, but its existence, is seen as confirmation of the theory, that despite international efforts to control CFCs, the ozone depletion problem will get worse before it gets better.